Crohn disease and ulcerative colitis constitute the major forms of chronic inflammatory bowel diseases. These disorders occur in young adults with an estimated prevalence of more than 1/1000 inhabitants in western countries. An initial panel of 25 European families with at least two affected siblings was typed at 270 highly polymorphic markers spanning the genome. Non-parametric sib pair linkage analysis showed two regions with p-values less than 0.01. A second panel of 53 families was typed for several markers in these two regions. One of the regions, the pericentromeric region of chromosome 16, was significant at the 0.01 level in the second panel. Taken together, multipoint linkage analysis of the two panels provide significant evidence for linkage to the chromosome 16 region (p < .00002). The locus appears to have a recessive mode of inheritance, a finding consistent with earlier segregation analyses. Further genotyping of the second panel is underway; at least one additional locus, dominant in mode of inheritance, is believed to be present in the second panel.